Modern communication standards sometimes require linear power amplification in contrast to, for example, frequency-modulation (FM) based systems for which highly non-linear transmitters and receivers were sufficient. Modern mobile communications standards such as UMTS, WiMAX and Long Term Evolution (LTE) are also intended to support a variety of different channel coding and modulation techniques (CDMA, OFDM, etc.), broader channel bandwidths and high peak-to-average power ratio (PAPR) modulation schemes. Moreover, there is an increasing demand for base stations supporting a wider range of frequency bands. Such base stations are more flexible and may be reconfigured if new frequency slots become available or a frequency slot gets assigned to a new communications standard, such as UMTS.
These developments place a high demand on the power amplifiers in the base transceiver station, in particular, with respect to linearity and bandwidth.
One approach to achieve good linearity in a transmitter is to digitally predistort a base band signal to achieve transmitter linearization in mobile radio infrastructure systems. Base band digital predistortion works well, but can prove to be costly to implement in a multi-element antenna-embedded radio application, due to the large number of predistorters required and the need to transmit high-bandwidth data to each transmitter. The data has a high bandwidth because the data must include the predistortion bandwidth and not just the wanted signal bandwidth. This latter problem, in particular, makes it difficult to achieve a wide transmission bandwidth with delta-sigma converters, whilst still maintaining the required signal-to-noise ratio in the output signal.
The International Application Publication WO 01/08294 A1 entitled “Transmission antenna array system with predistortion” assigned to Datum Telegraphic, Inc., discloses a transmission antenna array system comprising a plurality of amplification chains and a plurality of compensation circuits. A processing unit monitors input transmission signals and corresponding output signals of the plurality of amplification chains. The processing unit processes the input signals and the output signals to adaptively generate updates to the compensation parameters used by each of the plurality of compensation circuits. The compensation circuits are configured to digitally predistort the input transmission signal using a dynamically updated set of the compensation parameters.
U.S. Pat. No. 6,993,301 B1 in the name of Kenington et al., assigned to Andrew Corporation, is entitled “Lineariser for a signal handling apparatus”. The lineariser for reproducing distortion present in the output of an amplifier (or other signal handling device) generates a predistortion signal from an input to an amplifier. The predistortion signal is mixed into the amplifier input signal using, for example, a vector modulator. The predistortion signal may be derived in a quadrature format, to enable orthogonal components of the predistortion signal to be mixed into separate mixers of the vector modulator. The predistortion signal is generated digitally using a digital signal processor (DSP). U.S. Pat. No. 6,993,301 B1 discloses the linearisation of a single transmission path but not of an active antenna array.
U.S. Patent Application Publication No. 2009/0146736 A1 entitled “Baseband-derived RF digital predistortion” in the name of Kim et al, assigned to Dani System Co. Ltd., discloses a similar arrangement.